Hybrid Rocket Engine could Revolution Space Travel

Outside of the Culham Science Centre in Oxfordshire, England, sits a jet engine which engineer Alan Bond claims to be the beginning of the world’s first ever fully reusable spaceship capable of making cheap, reliable, regular trips to low Earth orbit. Bond states that “we’re looking at a revolution in transportation.”

The new spacecraft is referred to as the Skylon by Bond and his fellow engineers at Reaction Engines. It will look similar to the Concorde, take off like a conventional airplane, accelerate to Mach 5.2 in order to leave the atmosphere, and then upon re-entering the atmosphere it slows down and can land on the same runway it launched from.

Skylon. (PopSci)

The key to the Skylon is the Synergistic Air-Breathing Rocket Engine (Sabre) designed by Bond, a hybrid chemical rocket-jet engine. The Sabre can use oxygen from the air to fuel its engines, rather than needing external liquid oxygen tanks like the traditional space shuttle. This eliminates the need for disposable booster rockets, slashing the launch costs of any missions.

Mark Hempsell, future programs director at Reaction Engines, also explains that the lack of external tanks reduces the turn-around time between missions. “The Skylon could be ready to head back to space within two days of landing,” and cost as little as $10 million per mission. NASA’s space shuttle, on the other hand, took around two months to prepare for launch, and cost $100 million.

The Sklyon is impressive, and has so far attracted $92 million in combined investment from the British government and the European Space Agency (ESA), but further finance may be difficult to find. Reaction Engines intends to turn to public and private investors for the remaining $3.6 billion needed to complete the engine and run test flights within four years. Another $14 billion would be required to build the Skylon craft itself.

The challenge of creating a single-stage-to-orbit spaceship (SSTO) has vexed engineers for decades, but Bond’s idea to use a hybrid engine was the key to the current success. The main problem with using external oxygen in the atmosphere is that “by the time the plane hits Mach 2 or so, the air becomes very hot and extremely difficult to compress,” and finding a way of cooling it without adding extra weight was proving troublesome.

After years of designs and testing they managed to create a system that almost instantaneously cooled the air, allowing the engine to operate at higher speeds.

Reaction Engines explains how the Sabre engine works: Air traveling at Mach 5 enters the engine and passes through a heat exchanger. There, a network of paper-thin metal tubes filled with liquid helium chill the 2,000F air to –238F almost instantly. That chilled air flows into the turbocompressor, then into the thrust chambers, where it’s mixed with liquid hydrogen and ignited to produce thrust for the spacecraft.

Hempsell claims that the Skylon could potentially make 100 flights a year, enabling it to earn back all money spent in R&D and construction within a year. Its reliability and relative cheapness could also allow it to be used for supersonic aviation. Bond says that “it could enable an aircraft to fly anywhere in the world in under four hours.”